The rod-dominant, on-center bipolar cells are known to receive inputs not only from rods but also from cones. Our current study, using the carp retina, has been focused towards elucidation of the ionic mechanism of the response of this bipolar cell type to center and surround illuminations, with the following two main results. (1) Both the center depolarization and the surround hyperpolarization in the scotopic condition are regulated by changes in opposite directions of the conductance of Na ion-channels in the subsynaptic membrane, while those in the photopic condition by changes in opposite directions of the conductance of K ion- and/or Cl- -channels. (2) The Na ion-regulated surround hyperpolarization in the scotopic condition is reproduced by hyperpolarizing the rod-specific, intermediate horizontal cells by extrinsic current, while the K ion- and/or Cl- -regulated surround hyperpolarization in the photopic condition is reproduced by hyperpolarizing the cone-specific, external horizontal cells by extrinsic current. The result in (2) suggests that the surround hyperpolarization by light of this bipolar cell type is mediated by the hyperpolarization by light of these horizontal cells. The next step of our study is to deepen the analysis of the above observations and to extend similar approaches to other retinal cell types.